Marx's rarely used term for a transitional period after a revolution in which the working-class majority holds political power, before the state withers away in a communist society.
In more detail
Marx associated the idea with the Paris Commune's elected, recallable and modestly paid officials, and scholars read it as temporary rule by a democratic majority rather than a permanent dictatorship in the modern sense. Bakunin warned that it would become rule by a minority of educated leaders in the proletariat's name. Lenin's Bolsheviks used the term to justify one-party rule, and most social-democratic parties abandoned it.